does not exist. This is an important philosophical misconception about the nature of science and reasoning. Not knowing
the answer to a question in no way provides support that there is no answer. Only evidence in support of an alternative
explanation can do that.
12.6 SPECIATION: THE FORMATION OF NEW SPECIES
Section 12.6 Questions
(Page 576)
Understanding Concepts
1.
2.
3.
4.
5.
Answers will vary. Examples include the following:
(a) Nocturnal flying squirrels would not be able to breed with diurnal red squirrels.
(b) Many bird species have distinctive mating calls that prevent errors in mate selection.
(c) Lake trout live and breed in large deep lakes, while speckled (brook) trout live and breed in small fast-flowing
streams.
(d) Wind-pollinated plants have female flowers that are incompatible with the vast majorities of types of pollen that may
happen to land on them.
Many species are extremely similar in physical appearance but remain distinct species because of reproductive isolating
mechanisms that are not readily apparent such as temporal, behavioural, gametic, and ecological mechanisms.
Palaeontologists often only have evidence of macroscopic body parts and therefore are often unable to make judgments
about the numbers of species such fossils may really represent. The result is a “lower estimate” of species numbers. Of
course, palaeontologists can examine similar modern species and make estimates using them as models.
(a) gametic isolation
(b) ecological isolation
(c) behavioural isolation
(d) behavioural isolation
(e) gametic and/or mechanical isolation
(f) mechanical isolation
(g) zygotic mortality
(h) hybrid inviability
(i) temporal isolation
(j) hybrid infertility
Two likely causes are either zygotic mortality or gametic isolation. Other explanations include the possibility that one or
both adults are actually infertile hybrids themselves and not able to produce viable gametes. It is also possible that
behavioural isolation is keeping the individuals from successfully courting and mating.
Because these small populations are so easily and effectively isolated by roads, their individual populations may be
considered founder populations and be experiencing localized bottlenecks and/or extensive genetic drift. It is also possible
that differences in the micro and macro environments of these small areas are driving natural selection in different
directions in different locations, thereby reducing the genetic similarity among populations. This appears to be a case in
which allopatric speciation may be underway because of the isolation of the small populations.
Applying Inquiry Skills
6.
Isolated islands are more prone to unique forms of natural selection. The lack of gene flow among populations on remote
islands and populations on other lands masses causes the populations to evolve independently, and any changes in the
allele frequency in one location would not and could not be shared, resulting in genetic divergence over time. In PEI, gene
flow between the island and the mainland would reduce the potential for island populations to evolve distinctive traits.
7. (a) Those individuals that matured at a younger age or were smaller in stature would be selected for and have greater
reproductive success. This would lead to the evolution of a population of smaller deer.
(b) No. Mutations are random events. What will happen, however, is that any mutation that causes an individual to be
larger would be harmful and reduce the individual’s reproductive success, while, in contrast, any mutation that reduced
the size of the deer as adults would provide a selective advantage.
256 Unit 4
Evolution
Copyright © 2003 Nelson
Making Connections
8. (a) The canal may permit gene flow among closely related marine species on either side of the isthmus. This may result in
the formation of one species from two via crossbreeding. While it is unlikely that many marine fish would or could
actually pass through the lengthy canal, many species (especially invertebrates) might be carried on the outer surface of
ships or as stowaways in bilge water.
(b) By forming a physical barrier to the movement of some, especially small, terrestrial animals, the canal may cause
allopatric speciation to occur in many species.
9. Plant breeders form “hybrids” to combine the characteristics of two individuals—often these are the so-called F-1 hybrids
that are simply heterozygous individuals resulting from a cross of two “pure” breeding homozygous strains. In other cases
they may interbreed a cultivated species with a wild species to introduce pest resistance into the food crop. Wild barley
was used as a source of a gene that conferred resistance to powdery mildew. Entirely new food crops may also be
produced by hybridization. Triticale was produced by crossing wheat and rye. Often the hybrids are infertile. In the case
of the invasive weed purple loosestrife, ornamental hybrid varieties were grown in flower gardens in North America. This
European plant was thought to be infertile but proved to be able to produce viable seeds and escape into the wild. It is
now a serious invasive species in natural wetlands, out-competing many native species.
10. Human-created barriers include large dams, highways, canals (for terrestrial populations), urban sprawl, and farmland
dissecting former forested regions. Eliminating barriers include canals (for aquatic populations), air travel that removes
geographic barriers by intentionally or unintentionally moving one or more individuals across barriers such as oceans.
The consequences can be the elimination of species as hybridization occurs or species extinction through new
competition. In addition, new interactions may lead to the evolution of new of modified species. This has already occurred
with the introduction of many insect species.
INVESTIGATION 12.3.1 AGENTS OF CHANGE
(Pages 577–578)
Hypothesis
(a) Part I: We predict that allele frequencies will not change under the conditions of Part I.
Part II: We predict that the smaller the population sample we begin with, the more the allele frequencies will change.
Part III: We predict that the alleles in phenotypes that are favoured by natural selection will increase in frequency.
Experimental Design
(b) Student designs for Part II and III will vary. Sample data and experimental designs follow.
Part I sample data:
Trial
1
2
3
4
5
RR number and
frequency
4/0.2
5/0.25
4/0.2
6/0.3
6/0.3
Rr number and
frequency
12/0.6
9/0.45
11/0.55
10/0.5
11/0.55
rr number and
frequency
4/0.2
6/0.3
5/0.25
4/0.20
3/0.15
R allele number
and frequency
20/0.5
19/0.475
19/0.475
22/0.55
23/0.575
r allele number
and frequency
20/0.5
21/0.525
21/0.525
18/0.45
17/0.425
There was little change in allele frequencies over the “five generations.”
Part II sample design and data:
In Part II, students may choose to begin with a much smaller population size. Experimental design should include their initial
sample size (number of beads) and their sampling methods. They should also provide data entry tables to illustrate clearly
what data is being gathered and how it will be tabulated.
Copyright © 2003 Nelson
Chapter 12 Mechanisms of Evolution 257